1. Field of the Invention
The present invention relates to a device for detecting and indicating when the level of developer solution, held for use in an electrostatic photocopying machine in a tank, falls to a predetermined level. With only this level remaining, more solution should be added to the tank before the machine is used again to insure its proper operation.
Photocopying machines are now in widespread use for reproducing all forms of documents. These machines are usually either of the dry toner type or wet, liquid toner type. In the machines of the second type a copy sheet having a photoconductive coating is uniformly electrostatically charged. The image of a document to be copied is then projected onto the charged copy sheet while at an imaging station. A conductive ground plate, forming part of the imaging station, is effective to allow discharge of selected areas of the sheet in accordance with the projected image. In this way, a latent electrostatic image, corresponding to the image of the document, is formed on the sheet. The latent image is developed by passing the sheet through a tank containing a toner or developer solution that carries a large number of particles charged oppositely to the charge of the latent image. Accordingly, the particles are attracted to the image areas on the sheet to later be fixed and dried in a well known manner and thereby yield a finished photocopy. The developer solution usually comprises a toner concentrate, which includes the chargeable toner particles, suspended in a carrier, and a diluting carrier, which may be the same as or is otherwise miscible with the concentrate carrier. (Note that the "developer solution" may, in fact, be a suspension of toner particles in the carrier. However, as used in this specification and the concluding claims, the term "developer solution" is intended to generically mean any solution or suspension of opaque or substantially opaque toner material carried in a liquid vehicle and used to electrostatically develop an image.)
Toner particles are removed from the developer solution each time a copy is made. Therefore, if developer solution is not added to the holding tank periodically, acceptable copies will not be made. That is, for copies to be adequately developed, the developer solution should be maintained above a predetermined minimum level. However, a practical problem in determining when solution should be added to the photocopying machine exists. Specifically, the solution holding tank has a fixed capacity. Moreover, premixed developer solution is usually packaged in containers having a standard volume. In order to maintain the concentration of toner particles in the solution within acceptable limits, the entire contents of the storage container should be added to the solution tank at one time. Therefore, if solution is added before an amount equal to the contents of the container has been depleted from the tank, the tank may over flow.
Accordingly, it is desirable to have an indication of when solution in the tank has reached the predetermined minimum level, which will allow the photocopying machine to make acceptable copies but which is also low enough to permit addition of the contents of a solution container without overflowing.
2. Description of the Prior Art
Various techniques for indicating the level of developer solution in an appropriate holding tank is a photocopying machine have been proposed in the past. For example, a typical technique uses a float of buoyant material which actuates a read switch when the solution level falls to a predetermined minimum operating level. However, this technique does not offer sufficient accuracy for use in photocopying apparatus which utilize a solution tank that is relatively shallow and has a relatively large surface area. In such tanks small changes in the level of liquid in them result when large changes in the volume of liquid, compared to the total capacity of the tank, occur. Accurate solution level detection is also difficult to obtain because the tank is frequently removable from the photocopying apparatus to permit maintenance. However, when reinstalled the physical relationship between the tank and the apparatus may change resulting in further inaccuracy of or requiring recalibration of the solution level detector. Moreover, it is difficult to find a suitable buoyant material which is inert with respect to or at least resistant to the developer solution.
A device for indicating developer solution level in a photocopying apparatus is disclosed in U.S. Pat. No. 3,468,604 (Matkovich et al.), assigned to the assignee of the patent invention. This device includes a source of light focused, through separate transparent containers of toner concentrate and its carrier, on separate fiber optical probes that terminate outside of the photocopying apparatus housing or cabinet. Since the toner concentrate and the carrier are poor light transmitters, light does not reach the probes until the level of the liquid falls to a selected level.
However, the Matkovich et al., system is used to indicate liquid levels in generally vertically arranged containers that define relatively small liquid surface areas. Therefore, relatively large changes in the volume of liquid, compared to the capacity of the containers, cause relatively large changes in the liquid level which are easily detected. Accordingly, a high degree of systematic accuracy is not required to determine when liquid level has reached its acceptable minimum value.